This invention relates to the control arrangement used by a microprocessor contained in a disk drive controller adapted to be connected to up to four magnetic disk drives for controlling the drives. In order that up to four drives can be controlled, it is important that signals be passed both to and from the drive by the controller. That is, the controller must signal a drive, for example, when to be prepared to read or write data, while the drive must be enabled to signal the controller that it is, in fact, ready.
In a particular embodiment of interest, the disk drive comprises means for driving a so-called "floppy" disk contained within a rigid box-like cartridge. When the floppy disk is inserted into the disk drive, the drive opens a portion of the box so that the disk can be connected to a drive motor and so that the read/write head can be juxtaposed thereto. Details of the arrangements used to enable this are discussed in copending applications, Ser. No. 256,320 filed Apr. 22, 1981 and Ser. No. 256,594 filed Apr. 22, 1981. The disks within cartridges are designed to be readily replaceable by the operator of the disk drive upon his seeking to employ varying ones of said disks and accordingly, it is important that a signal indicating the operator has completed replacement of the disk within a given drive be likewise passed to the controller.
In the embodiment with which the invention is particularly concerned, the floppy disk is juxtaposed in operation to a so-called Bernoulli plate. When the drive is energized and the floppy disk begins to spin, an air cushion is formed between it and the Bernoulli plate, such that the disk is substantially rigid at design speed. This permits the use of a "flying" or "Winchester" read/write head to be closely juxtaposed to the disk, which allows higher density reading and writing than in the floppy disk prior art. Since the head flies on an air bearing above the floppy disk surface, there is substantially no friction therebetween. However, if the head remains in a single position with respect to the disk for long periods of time it is possible that local conditions may cause the disk or head to suffer some damage. Accordingly, it would be desirable to provide means for stopping the motor upon the detection of a period of lack of read/write activity longer than some predetermined time. Prior art timing techniques, which would typically be incorporated in the drive, are not sufficient to perform this time measurement function with accuracy.